An axial-flow fan of a prior design is shown in German Patent De-Al No. 25 51 615.
An axial-flow fan mounted within a frame requires a connecting link or links between the outside frame, which surrounds the impeller, and the drive motor, which is mounted at the center. This connecting link, hereinafter called "mounting portion," provides a central mounting of the drive motor; and also provides support for the electric leads or wires connecting to the electric motor.
Aerodynamically, these mounting portions present an air flow resistance which causes both a loss of flow efficiency and additional noise. Therefore, the cross section of these members is kept as small as possible. However, since they are usually located very close to the blades (inlet or outlet side), the flow velocities are relatively high in this area. As the blades pass by, they produce a sound (interfering body near the blade) whose frequency, sound pressure and bandwidth (frequency bandwidth) are determined mainly by the number of blades, fan speed, blade geometry, shape and position of the mounting portions, number of the mounting portions, and distance between mounting portion and blade.